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Why do volcanoes erupt in different ways?

Most volcanoes occur on plate boundaries. Plate boundaries
are areas where Earth's shifting plates meet or split apart,
usually with violent results.

California's
Mt. Shasta
is a stratovolcano.

Plate margins that are coming together are called convergent
margins, while those that are splitting apart are called divergent.
A third type, transform-fault margins, are sliding against
each other, going in opposite directions (like those of the
San Andreas Fault). Volcanoes can occur on convergent or divergent
plate margins or over a hotspot, a spot inside the mantle
that heats an area of the plate above it.

Colliding Plates
Along convergent margins, when two plates meet, sometimes one descends, usually of oceanic composition, beneath the other, usually of continental composition, in a process called subduction. As the descending plate is forced deeper into the mantle, parts of it begin to melt and form magma that rises to the surface, often in explosive eruptions. Subduction zones tend to create large, classic, cone-shaped volcanoes called stratovolcanoes, such as Mt. St. Helens in Washington State, or Mt. Shasta in California.

Separating Plates
At divergent margins, plates are coming apart and hot rock
forces its way to the surface. Many divergent plate margins
are under the oceans, creating long undersea rift zones that
fill with lava. In some eruptions at divergent margins, the
relatively calm, smooth flow of lava creates volcanoes with
gently sloping sides, called shield volcanoes.

Hotspots
Hotspots can also cause shield volcanoes to form. As plates
move over hotspots, volcanoes spring up and die down in turn,
often creating an island chain. The Hawaiian Islands are the
result of a hotspot.